1. Field of the Invention
The present invention relates to a method of and apparatus for setting and maintaining a desired holding force between a pair of relatively movable members that are adapted to be brought into and out of contact with each other and while in contact are held together with a predetermined holding force. More particularly, the present invention relates to the structure and operation of a mold clamping system in a plastics injection molding machine, to permit automatic setting of the parts of a mold clamping system to provide a desired mold clamping force, and, additionally, to periodically check the positions of parts of the clamping system after a predetermined number of clamp operating cycles to determine whether readjustment of the positions of parts is necessary to maintain a desired mold clamping force.
2. Description of the Related Art
Various types of mold clamping arrangements have been developed to hold a pair of mold members together while molten plastics materials are injected into a mold cavity defined by the closed mold members. Because of the high injection pressures employed to inject molten plastic material into a mold cavity, of the order of thousands of pounds per square inch, a high mold clamping force is normally required, on the order of several hundred tons or greater, in order to resist the force resulting from the internal pressure within the mold from forcing the mold members apart, which could result in a thin, external tab or protrusion of plastic material on the molded article, commonly referred to as "flash," or in incomplete filling of the mold cavity as a result of excessive separation of the mold members during injection. In either case, the resulting molded part is unacceptable.
Although it would be possible to avoid the mold separation problem by consistently imposing a very high mold clamping force to hold a pair of molds together, regardless of the injection pressure of the plastic material, repeated operation with unnecessarily high clamping loads could result in damage to the molds and is therefore undesirable. Accordingly, the externally applied mold clamping force should bear some close relationship with the actual internal mold separation force generated within the mold cavity, and therefore close control over the mold clamping force is highly desirable.
A method and apparatus for controlling the mold clamping force imposed upon the molds in an injection molding machine were disclosed by Ulrich Eggenberger et al. in U.S. Pat. No. 3,642,405, which issued on Feb. 15, 1972, and in U.S. Pat. No. 3,819,774, which issued on June 25, 1974. Those patents show an injection molding machine having a toggle-type mold clamping arrangement wherein the normally stationary clamp support platen that supports the toggle clamping system is selectively movable toward or away from the stationary mold platen that supports one of the mold members, in order to attempt to maintain a predetermined constant mold clamping force between the mold members over a number of machine operating cycles. The position of the clamp platen is adjusted relative to the stationary mold platen to adjust for thermal expansion and other effects and to maintain the desired mold clamping force.
Another arrangement for maintaining a predetermined desired mold clamping force is disclosed in U.S. Pat. No. 4,281,977, which issued on Aug. 4, 1981, to Robert E. Farrell, and in U.S. Pat. No. 4,301,100, which issued on Nov. 17, 1981, also to Robert E. Farrell. Although directed to achieving a similar result as in the Eggenberger et al. patents referred to above, the Farrell patents disclose a system in which the adjustment to maintain a desired clamping force is effected by adjusting the length of the tie rods by means of a high ratio mechanical drive, and by utilizing a displacement sensor that is connected with the high ratio mechanical drive to effect any necessary adjustment to maintain the desired mold clamping force as the machine operates through a predetermined number of operating cycles.
Another patent that discloses an arrangement for maintaining a predetermined mold clamping force in an injection molding machine that includes a toggle-type mold clamping system is U.S. Pat. No. 4,832,884, which issued May 23, 1989, to Ruedi Speck et al. That patent also incorporates a drive arrangement for moving a normally stationary clamp platen relative to a stationary mold platen, but it also senses mold clamping force by providing strain gauges on each toggle lever to measure the compressive stresses generated in the toggle levers during operation of the mold clamping system. Those forces are averaged and the averages are monitored over several operating cycles. When the sensed forces exceed a predetermined tolerance from a base value, an adjustment is made to adjust the mold spacing as necessary to maintain the desired mold clamping force value.
Although methods and apparatus have been suggested for maintaining a predetermined mold clamping force in an injection molding machine, as reflected in the above-identified patents, none of those patents teaches an arrangement for initially automatically setting the mold clamping system to produce a desired mold clamping force by initially setting up the spacing of the molds and the parts of the mold clamping system to achieve a desired mold clamping force.
It is an object of the present invention to overcome the limitations of the prior art devices and to provide an automatic setup arrangement to provide a mold spacing and mold clamp operation that will result in the mold desired clamping force without the need for manual setting of the respective parts.
It is another object of the present invention to provide a mold clamping control arrangement whereby after an initial setting of the parts of the mold clamping system machine has been established to obtain a desired mold clamping force, the clamping system is monitored during subsequent operating cycles, and changes are made to the relative positions of the parts to maintain the desired mold clamping force.
It is a still further object of the present invention to provide a mold clamping system that is fully automatic in setup and operation and that does not require the intervention of an human operator.